2.1 Influenza Virus
Virus families containing enveloped single-stranded RNA of the negative-sense genome are classified into groups having non-segmented genomes (Paramyxoviridae, Rhabdoviridae, Filoviridae and Borna Disease Virus) or those having segmented genomes (Orthomyxoviridae, Bunyaviridae and Arenaviridae). The Orthomyxoviridae family, described in detail below, and used in the examples herein, includes the viruses of influenza, types A, B and C viruses, as well as Thogoto and Dhori viruses and infectious salmon anemia virus.
The influenza virions consist of an internal ribonucleoprotein core (a helical nucleocapsid) containing the single-stranded RNA genome, and an outer lipoprotein envelope lined inside by a matrix protein (M1). The segmented genome of influenza A virus consists of eight molecules (seven for influenza C) of linear, negative polarity, single-stranded RNAs which encode eleven polypeptides, including: the RNA-dependent RNA polymerase proteins (PB2, PB1 and PA) and nucleoprotein (NP) which form the nucleocapsid; the matrix membrane proteins (M1, M2); two surface glycoproteins which project from the lipid containing envelope: hemagglutinin (HA) and neuraminidase (NA); the nonstructural protein (NS1), nuclear export protein (NEP); and the proapoptotic factor PB1-F2. Transcription and replication of the genome takes place in the nucleus and assembly occurs via budding on the plasma membrane. The viruses can reassort genes during mixed infections.
Influenza virus adsorbs via HA to sialyloligosaccharides in cell membrane glycoproteins and glycolipids. Following endocytosis of the virion, a conformational change in the HA molecule occurs within the cellular endosome which facilitates membrane fusion, thus triggering uncoating. The nucleocapsid migrates to the nucleus where viral mRNA is transcribed. Viral mRNA is transcribed by a unique mechanism in which viral endonuclease cleaves the capped 5′-terminus from cellular heterologous mRNAs which then serve as primers for transcription of viral RNA templates by the viral transcriptase. Transcripts terminate at sites 15 to 22 bases from the ends of their templates, where oligo(U) sequences act as signals for the addition of poly(A) tracts. Of the eight viral RNA molecules so produced, six are monocistronic messages that are translated directly into the proteins representing HA, NA, NP and the viral polymerase proteins, PB2, PB1 and PA. The other two transcripts undergo splicing, each yielding two mRNAs which are translated in different reading frames to produce M1, M2, NS1 and NEP. The PB1 segment encodes a second protein, the nonstructural PB1-F2 protein, by using of an alternative ATG. In other words, the eight viral RNA segments code for eleven proteins: nine structural and two nonstructural. A summary of the genes of the influenza virus and their protein products is shown in Table I below.
TABLE IINFLUENZA VIRUS GENOME RNA SEGMENTS AND CODINGASSIGNMENTSaLengthdLengthbEncoded(AminoMoleculesSegment(Nucleotides)PolypeptidecAcids)Per VirionComments12341PB275930-60RNA transcriptase component;host cell RNA cap binding22341PB175730-60RNA transcriptase component;initiation of transcriptionPB1-F287Proapoptotic factor32233PA71630-60RNA transcriptase component41778HA566500Hemagglutinin; trimer; envelopeglycoprotein; mediatesattachment to cells51565NP4981000Nucleoprotein; associated withRNA; structural component ofRNA transcriptase61413NA454100Neuraminidase; tetramer;envelope glycoprotein71027M12523000Matrix protein; lines inside ofenvelopeM296?Structural protein in plasmamembrane; spliced mRNA8890NS1230Nonstructural protein; functionunknownNEP121?Nuclear export protein; splicedmRNAaAdapted from R. A. Lamb and P. W. Choppin (1983), Annual Review of Biochemistry, Volume 52, 467-506.bFor A/PR/8/34 straincDetermined by biochemical and genetic approachesdDetermined by nucleotide sequence analysis and protein sequencing
The pathogenicity of influenza viruses is modulated by multiple virus and host factors. Among the host factors that fight virus infections, the type I interferon (IFNα/β) system represents a powerful antiviral innate defense mechanism which was established relatively early in the evolution of eukaryotic organisms (Garcia-Sastre, 2002, Microbes Infect 4:647-55). The antiviral IFNα/β system involves three major steps: (i) detection of viral infection and IFNα/β secretion, (ii) binding of IFNα/β to its receptors and transcriptional induction of IFNα/β-stimulated genes, and (iii) synthesis of antiviral enzymes and proteins. Most viruses, however, have acquired specific genetic information encoding IFNα/β antagonist molecules, which effectively block one or more steps of the antiviral IFNα/β system. Influenza A viruses express a non-structural protein in infected cells, the NS1 protein (described in detail, infra), which counteracts the cellular IFNα/β response (Garcia-Sastre et al., 1998, Virology 252:324-30).
The influenza A virus genome contains eight segments of single-stranded RNA of negative polarity, coding for two nonstructural and nine structural proteins. The nonstructural protein NS1 is abundant in influenza virus infected cells, but has not been detected in virions. NS1 is a phosphoprotein found in the nucleus early during infection and also in the cytoplasm at later times of the viral cycle (King et al., 1975, Virology 64: 378). Studies with temperature-sensitive (ts) influenza mutants carrying lesions in the NS gene suggested that the NS1 protein is a transcriptional and post-transcriptional regulator of mechanisms by which the virus is able to inhibit host cell gene expression and to stimulate viral protein synthesis. Like many other proteins that regulate post-transcriptional processes, the NS1 protein interacts with specific RNA sequences and structures. The NS1 protein has been reported to bind to different RNA species including: vRNA, poly-A, U6 snRNA, 5′ untranslated region as of viral mRNAs and ds RNA (Qiu et al., 1995, RNA 1: 304; Qiu et al., 1994, J. Virol. 68: 2425; Hatada Fukuda 1992, J Gen Virol. 73:3325-9. Expression of the NS1 protein from cDNA in transfected cells has been associated with several effects: inhibition of nucleo-cytoplasmic transport of mRNA, inhibition of pre-mRNA splicing, inhibition of host mRNA polyadenylation and stimulation of translation of viral mRNA (Fortes, et al., 1994, EMBO J. 13: 704; Enami et al., 1994, J. Virol. 68: 1432; de la Luna et al., 1995, J. Virol. 69:2427; Lu et al., 1994, Genes Dev. 8:1817; Park et al., 1995, J. Biol Chem. 270, 28433; Nemeroff et al., 1998, Mol. Cell. 1:1991; Chen et al., 1994, EMBO J. 18:2273-83). In particular, the NS1 protein has three domains that have been reported to have a number of regulatory functions during influenza virus infection. The amino-terminal 73 amino acids are responsible for binding to RNAs (Qian et al., 1995, RNA 1:948-956), particularly double stranded RNAs, conferring to the virus the ability to escape the interferon α/β response (Donelan et al., 2003, J. Virol. 77:13257-66). The central portion of the protein interacts with the eukaryotic translation initiation factor 4GI facilitating preferential translation of viral mRNAs over host mRNAs (Aragón et al., 2000, Mol. Cell Biol. 20:6259-6268). The carboxy-terminus or the effector domain has been shown to inhibit host mRNA processing, specifically, inhibition of host mRNA polyadenylation (Nemeroff et al., 1998, Mol. Cell 1:991-1000), binding to poly(A) tails of mRNA inhibiting nuclear export (Qiu and Krug, 1994, J. Virol. 68:2425-2432) and inhibition of pre-mRNA splicing (Lu et al., 1994, Genes Dev. 8:1817-1828).
Studies of human recombinant influenza virus lacking the NS1 gene (delNS1) showed that this virus could only replicate in IFN-incompetent systems such as STAT1−/− mice or Vero cells; thus the NS1 protein is responsible for IFN antagonist activity (Garcia-Sastre et al., 1998, Virology 252:324-330). Also, it has been shown that human influenza viruses with truncated NS1 proteins are attenuated in mice (Egorov et al., 1998, J. Virol. 72:6437-6441) and provide protection against wild-type challenge (Talon et al., 2000, Proc. Natl. Acad. Sci. USA 97:4309-4314).
2.2 Swine Influenza Virus
Swine influenza (SI) is an acute respiratory disease of swine caused by type A influenza viruses. Its severity depends on many factors, including host age, virus strain, and secondary infections (Easterday, 1980, Philos Trans R Soc Lond B Biol Sci 288:433-7). Influenza A viruses are segmented negative-strand RNA viruses and can be isolated from a number of other animal host species, including birds, humans, horses, whales, and mink. Although whole influenza viruses rarely cross the species barrier, gene segments can cross this barrier through the process of genetic reassortment, or genetic shift. Since pigs support the replication of both human and avian influenza A viruses (Kida et al., 1994, J Gen Virol 75:2183-8), they have been postulated to play an important role in interspecies transmission by acting as a “mixing vessel” for reassortment between viruses specific to different host species (Scholtissek, 1994, Eur J Epidemiol 10:455-8). This may lead to the generation of novel influenza viruses capable of crossing the species barrier to humans. There are three subtypes of SI viruses (SIV) currently circulating in pigs in the U.S.: H1N1, H3N2, and H1N2 (Olsen, 2002, Virus Res 85:199-210; Karasin et al., 2002, J Clin Microbiol 40:1073-9; Karasin et al., 2000, Virus Res 68:71-85; Olsen et al., 2000, Arch Virol 145:1399-419; Webby et al., 2000, J Virol 74:8243-51; Webby et al., 2001, Philos Trans R Soc Lond B Biol Sci 356:1817-28; Zhou, 2000, Vet Microbiol 74:47-58). Before 1998, mainly “classical” H1N1 SIVs were isolated from swine in the United States (Kida et al., 1994, J Gen Virol 75:2183-8; Scholtissek, 1994, Eur J Epidemiol 10:455-8; Olsen et al., 2000, Arch Virol. 145:1399-419). In 1998, SIVs of the subtype H3N2 were isolated in multiple states in the United States. These viruses were generated by reassortment between human, avian and classical swine viruses, they are undergoing rapid evolution and in general they cause more severe disease than classical H1N1 SIV.
Pathogenicity of influenza viruses is modulated by multiple virus and host factors. Among the host factors that fight virus infections, the type I interferon (IFNα/β) system represents a powerful antiviral innate defense mechanism which was established relatively early in the evolution of eukaryotic organisms (Garcia-Sastre, 2002, Microbes Infect 4:647-55). The antiviral IFNα/β system involves three major steps: (i) detection of viral infection and IFNα/β secretion, (ii) binding of IFNα/β to its receptors and transcriptional induction of IFNα/β-stimulated genes, and (iii) synthesis of antiviral enzymes and proteins. Most viruses, however, have acquired specific genetic information encoding IFNα/β antagonist molecules, which effectively block one or more steps of the antiviral IFNα/β system. Influenza A viruses express a non-structural protein in infected cells, the NS1 protein, which counteracts the cellular IFNα/β response (Garcia-Sastre et al., 1998, Virology 252:324-30).
Influenza infection in pigs was first reported in 1918 and the first swine influenza viruses were isolated from pigs in 1930 (Shope, R. E., 1931, J. Exp. Med. 54:373-385). These first isolates were the progenitors of what is recognized as the H1N1 lineage of swine influenza A viruses. From 1930 to the 1990s, influenza in North American pigs was caused almost exclusively by infection with H1N1 swine viruses. A dramatic shift in the pattern of swine influenza began around 1997, when an unexpected and substantial increase in H3 seropositivity (8%) was detected, and H3N2 viruses began to be isolated from pigs in both the US and Canada (Olsen et al., 2000, Arch. Virol. 134:1399-1419). Furthermore, reassortment between H3N2 viruses and H1N1 swine viruses resulted in the detection of second generation H1N2 reassortant viruses (Karasin et al., 2000, J. Clin. Microbiol. 38:2453-2456; Karasin et al., 2002, J. Clin Microbiol. 40:1073-1079). In addition, avian H4N6 viruses of duck origin have been isolated from pigs in Canada (Karasin et al., 2000, J. Virol. 74:9322-9327). The generation of these novel viruses in addition to the described antigenic drift variants of H1N1 swine influenza viruses, introduce potential veterinary and human public health implications.
In 1998, a new strain of swine influenza virus to which pigs had little immunity sickened every pig in an operation of 2400 animals. Although there has been only one influenza subtype which has sickened North American pigs since 1930, in the last few years a quick succession of new flu viruses has been sweeping through North America's 100 million pigs. After years of stability, the North American swine flu virus has jumped onto an evolutionary fast track, bringing out variants every year. This has had not only an undesired effect on the farming industry and a negative economic impact, but, there is also concern by experts that the evolving swine flu increases the likelihood that a novel virus will arise that is transmissible among humans. Fortunately, the new pig strains that have appeared in North America so far do not appear to readily infect humans.
2.3 Attenuated Viruses
Inactivated virus vaccines are prepared by “killing” the viral pathogen, e.g., by heat or formalin treatment, so that it is not capable of replication. Inactivated vaccines have limited utility because they do not provide long lasting immunity and, therefore, afford limited protection. An alternative approach for producing virus vaccines involves the use of attenuated live virus vaccines. Attenuated viruses are capable of replication but are not pathogenic, and, therefore, provide for longer lasting immunity and afford greater protection. However, the conventional methods for producing attenuated viruses involve the chance isolation of host range mutants, many of which are temperature sensitive; e.g., the virus is passaged through unnatural hosts, and progeny viruses which are immunogenic, yet not pathogenic, are selected.
A conventional substrate for isolating and growing influenza viruses for vaccine purposes are embryonated chicken eggs. Influenza viruses are typically grown during 2-4 days at 37° C. in 10-12 day old eggs. Although most of the human primary isolates of influenza A and B viruses grow better in the amniotic sac of the embryos, after 2 to 3 passages the viruses become adapted to grow in the cells of the allantoic cavity, which is accessible from the outside of the egg (Murphy, B. R., and R. G. Webster, 1996. Orthomyxoviruses pp. 1397-1445. In Fields Virology. Lippincott-Raven P. A.).
Recombinant DNA technology and genetic engineering techniques, in theory, would afford a superior approach to producing an attenuated virus since specific mutations could be deliberately engineered into the viral genome. However, the genetic alterations required for attenuation of viruses are not known or predictable. In general, the attempts to use recombinant DNA technology to engineer viral vaccines have mostly been directed to the production of subunit vaccines which contain only the protein subunits of the pathogen involved in the immune response, expressed in recombinant viral vectors such as vaccinia virus or baculovirus. More recently, recombinant DNA techniques have been utilized in an attempt to produce herpes virus deletion mutants or polioviruses which mimic attenuated viruses found in nature or known host range mutants. Until 1990, the negative strand RNA viruses were not amenable to site-specific manipulation at all, and thus could not be genetically engineered.
Although these viruses are beneficial because they are immunogenic and not pathogenic, they are difficult to propagate in conventional substrates for the purposes of making vaccines. Furthermore, attenuated viruses may possess virulence characteristics that are so mild as to not allow the host to mount an immune response sufficient to meet subsequent challenges.
Human influenza viruses does not replicate efficiently in birds, and vice versa due to differences in receptors which bind the viruses. In contrast, pigs are uniquely susceptible to infection with human and avian viruses because they possess receptor types present both in humans and avian influenza viruses. As a result, pigs have been hypothesized to be the “mixing vessel” hosts for human-avian virus reassortment and there is support for this theory from several studies. See, e.g., Shu et al., 1994, Virology 202:825-33; Scholtissek, 1990, Med. Principles Pract. 2:65-71; Zhou et al., 1999 J Virol. 73:8851-6. This mixing facilitates the generation of novel human influenza virus strains and the initiation of influenza pandemics.
Inactivated or “killed” influenza virus preparations are the only influenza vaccines currently licensed in the United States. An alternative approach for producing virus vaccines to the inactivated virus vaccines in which the viral pathogen is “killed”, involves the use of attenuated live virus vaccines which are capable of replication but are not pathogenic. Live vaccines which are administered intranasally may have advantages over their inactivated counterparts. Firstly, live vaccines are thought to induce improved cross-reactive cell-mediated cytotoxicity as well as a humoral antibody response, providing better protection than inactivated vaccines (Gorse and Belshe, 1990, J. Clin. Microbiol. 28:2539-2550; and Gorse et al., 1995, J. Infect. Dis. 172:1-10). Secondly, live vaccines also have the advantage of intranasal administration which avoids the swelling and muscle soreness occasionally associated with the intramuscular administration of inactivated adjuvanted vaccines. These live vaccines have been reported to induce not only humoral responses against homotypic influenza virus but also crossreactive cell-mediated cytotoxicity. Further advantages of live vaccines include the ease of intranasal administration, induction of mucosal immunity, longer lasting immunity, and its cost effectiveness. These are all important considerations regarding potential swine influenza vaccines.
Thus, new and more effective vaccines and immunogenic formulations for preventing swine influenza virus infections generated by such technology are needed.